Most of the conventional nonwoven fabrics were random nonwoven fabrics, so that most of them were not strong enough and poor in dimensional stability. In order to improve the disadvantages involved in these conventional nonwoven fabrics, the present inventors have proposed several methods each for manufacturing a nonwoven fabric by stretching fiber webs and crosswise laminating them together (Japanese Patent Publication No. Hei 3-36948, Japanese Patent Laid-Open Publication No. Hei 2-269859, Japanese Patent Laid-Open Publication No. Hei 2-269860). The present invention is the one which has been accomplished by improving and developing further these inventions.
Heretofore, a variety of examples have been known that mixed spun filaments or conjugate spun filaments prepared by employing polymers of different kinds are applied to the manufacturing of a nonwoven fabric.
For instance, bulky conjugate filaments are disclosed in Japanese Patent Laid-Open Publication No. Hei 4-24216 (short fiber nonwoven fabric), Japanese Patent Laid-Open Publication No. Hei 2-182963 (spunbonded nonwoven fabric), and Japanese Patent Laid-Open Publication No. Hei 4-41762 (spunbonded nonwoven fabric); adherent conjugate filaments are disclosed in Japanese Patent Laid-Open Publication No. Hei 2-61156 (spunbonded nonwoven fabric) and Japanese Patent Laid-Open Publication No. Hei 4-316608 (spunbonded nonwoven fabric); mixed filaments are disclosed in Japanese Patent Laid-Open Publication No. Hei 3-269154 (spunbonded nonwoven fabric); and the water-jet processing of nonwoven fabric composed of conjugate filaments is disclosed in Japanese Patent Laid-Open Publication No. Hei 4-316608 (spunbonded nonwoven fabric).
The above described conventional nonwoven fabrics involve the ones in which conjugate filaments and polymer filaments of different kinds are used together. However, they are short fiber nonwoven fabrics which are made by cutting conjugate filaments and polymer filaments of different kinds, so that, although the bulkiness is satisfactory, the strength and dimensional stability are not good. Furthermore, there have been long filament nonwoven fabrics such as spunbonded nonwoven fabric or melt-blown nonwoven fabric in accordance with conjugate spinning. However, since these nonwoven fabrics are not stretched, the effect of shrinkage cannot be produced, so that their bulkiness is insufficient and strength as well as dimensional stability are not satisfactory.
More specifically, in these nonwoven fabrics according to the prior art, the balance among the bulkiness, the strength as a single filament, and the strength as the whole material of nonwoven fabric is insufficient, so that they cannot not have physical properties enough as the ones which can be employed in place of the woven fabrics. In addition to the above, when a basis weight is small (e.g., less than 20 g/m.sup.2) in conventional nonwoven fabrics, the uniformity of the resulting nonwoven fabric is inferior in general, so that such nonwoven fabrics could not be used in a field where the comparable dimensional stability to that of woven fabric is required, because of the reason that conventional nonwoven fabrics have insufficient strength in addition to the inferior uniformity as described above.
Meanwhile, the crosswise laminated nonwoven fabrics disclosed in the above inventions of us is prepared by bonding fiber webs with emulsion adhesive or thermally embossing bonding operation, so that refined feeling and softness are sometimes insufficient as nonwoven fabrics.
In order to make better the above described various disadvantages involved in the conventional nonwoven fabrics, the present inventors proposed some inventions in which nonwoven fabrics are stretched and suitably laminated to produce a new kind of nonwoven fabrics (Japanese Patent Publication No. 3-36948, Japanese Patent Laid-Open Publication No. 2-269859, Japanese Patent Laid-Open Publication No. 2-269860, etc.)
As described above, it is demanded to provide such a nonwoven fabric having strength equivalent to a woven fabric as well as being excellent in softness, puffiness (low bulk density), and having high ductility and good touch feeling. It is also desired that a nonwoven fabric has a uniformity, because its practical value is lost if the uniformity in the basis weight is poor in a high strength nonwoven fabric.
An object of the present invention is to develop a nonwoven fabric which can be used suitably for the utilities most equal to those of woven fabrics such as disposable clothing, base fabrics for synthetic leather or artificial leather, which could not be attained hitherto with the conventional nonwoven fabrics, with the addition of characteristic features including strength as well as uniformity, good touch feeling, bulkiness and thinness.
Moreover, another object of the present invention is to provide a nonwoven fabric which can be used suitably for applications of packaging materials, construction materials or else, which fabric have a high value in the biaxial work of rupture (which will be described later) which value could not obtained in the conventional nonwoven or woven fabrics, so that the resulting nonwoven fabric is thin and economically used for the above purposes.
In addition, the nonwoven fabric must be produced inexpensively and it has various uses, so that small quantities of nonwoven fabrics in many kinds are must be produced. In this respect, according to conventional manufacturing processes, it was difficult to produce a nonwoven fabric which is particularly excellent in both the strength and bulkiness at low cost.
Accordingly, it is desired to provide a novel method for manufacturing a nonwoven fabric in which method a highly improved bulkiness and good touch feeling which are characteristic to the nonwoven fabric can be realized together with the solution to the above described problems in strength, uniformity and dimensional stability of nonwoven fabrics. Moreover, it is desired that the method is suitable for producing many kinds of product in relatively small quantities without losing the advantage in economy.